Conversation

#XMPP (aka #Jabber) is an easy, widely used free federated chat system.

There are thousands of instances to sign up at, such as https://chinwag.im and
https://www.xmpp.jp

There are also *lots* of apps.

Maybe the most important thing is that an app supports OMEMO privacy:

https://omemo.top

Some recommended OMEMO-compatible apps:

Conversations (Android)
https://conversations.im

Gajim (Win/Linux)
https://gajim.org

Monal (iOS/Mac)
https://monal.im

#Alternatives